Known as a legged mobile robot, particularly as a legged mobile robot with two legs, is a robot described in, for example, Japanese Patent Laid-Open Publication No. Showa 62-97005 and Japanese Patent Laid-Open Publication No. Showa 63-150176. Control of robots including the legged mobile robot is detailed in, for example, Robotics Handbook, Robotic Society of Japan, Oct. 20, 1990.
The stability of the legged mobile robot, particularly the legged mobile robot with two legs, is intrinsically low. Therefore, when disturbance or the like is applied to the robot, the posture of the robot becomes unstable. Thus, it is preferred that an inclination angle (posture angle) of the body of the robot and the rate of change of the angle are detected and the posture of the robot is then controlled in accordance with the detected values. To do so, control was earlier proposed by the applicant in Japanese Patent Laid-Open Publication No. Heisei 4-201187 and the like. In this control, a plurality of candidate landing positions are prepared in advance, and any one of those candidate landing positions is selected in a control cycle of each step and then a desired gait is switched.
However, in this control, even when posture is disrupted, there occurs no effect of recovering the posture until the next landing. Therefore, the posture may be largely disrupted before the next landing and the robot may fall over. In addition, this control has the following disadvantage. Even though floor contactability is diminished, as a result of disruption of the posture which causes the ZMP (the center of gravity where vertical floor reaction force acts and a horizontal component of a floor reaction force's moment becomes zero; zero moment point) to shift close to the vicinity of the boundary of a range where the ZMP can exist, the ZMP remains shifted to the vicinity of the boundary until the posture is recovered by correcting the gait in the next step. Thus, contact with the ground cannot be recovered until then.
Thus, the applicant has proposed the following technology in, for example, Japanese Patent Laid-Open Publication No. Heisei 5-305579. In this technology, a desired gait is generated, in which a floor reaction force is set to ensure a state of dynamic equilibrium, based on a dynamic model of a robot. When errors occur between the desired and actual values of the inclination angles and inclination angular velocities due to a modeling error or disturbance, displacement or a moment of the floor contacting portion of the actual robot is deliberately shifted from that of a reference gait. Accordingly, the floor reaction force of the actual robot (actual floor reaction force) is deliberately shifted from the floor reaction force of the desired gait which is calculated by the model, thus obtaining a recovering force which brings inclination of the actual robot near inclination of the model. Note that, the “floor reaction force” herein means a resultant force and resultant moment acting at a certain point of action, obtained as the sum of all floor reaction forces acting on respective legs.
However, when this technology is used, during a one-leg supporting period of a biped mobile robot, for example, it is sometimes infeasible to generate the total floor reaction force with the ZMP outside of the contact surface, even though the ankle joint angle is largely shifted to deliberately and greatly shift the floor reaction force of actual robot. Therefore, a part of the foot rises off the floor. Hence, when the actual robot greatly inclines relative to the model, it is sometimes impossible to obtain the sufficient recovering force for bringing inclination of actual robot back to normal. In this case, the actual robot falls over. In other words, with the technology proposed in Japanese Patent Laid-Open Publication No. Heisei 5-305579, the recovering effect reducing the errors between the desired values and actual values is limited within the range where the ZMP can exist.
The above technologies are explained with reference to the drawings. As for the control which causes joint displacement of the actual robot to strictly follow the model, the ZMP of the actual robot shown by a solid line in FIG. 61 coincides with the desired ZMP determined by the model, if the inclination angle and inclination angular velocity of the actual robot coincide with those of the model. Thus, the robot walks with a desired posture. In reality, however, the errors in the inclination angles and inclination angular velocities occur as shown by a dotted line, due to a modeling error or disturbance as mentioned earlier. In addition, the gravitational effect causes these shifts to have a tendency toward divergence in which the shifts increase more as the errors increase in magnitude. Once the actual ZMP shifts from the desired ZMP, a moment M obtained by multiplying a distance x between the actual and desired ZMPs by a force F is generated about the desired ZMP, as shown in FIG. 62. In other words, a desired moment can be generated by controlling joint displacement to deliberately shift the actual ZMP from the desired ZMP. Based on this idea, the earlier proposed technology generates a moment in a direction for posture recovery by deliberately shifting the actual ZMP as shown in FIG. 62 when the errors in inclination (angle and angular velocity) occur. When this technology is used, however, the range in which the actual ZMP can be shifted is limited within a foot region Xsole, as shown in the drawing. Therefore, the moment which can be generated in the direction of posture recovery is limited, and thus the posture recovery of the robot may not be pursued appropriately.
Thus, the applicant of the application concerned has also proposed the following technology in Japanese Patent Laid-Open Publication No. Heisei 5-337849 or the like. Specifically, in this technology, when the errors in inclination of the robot occur, the actual ZMP is shifted as described above to generate a moment in the direction of posture recovery about the desired ZMP within allowable range, and at the same time, movement of a desired gait is determined so as to generate a moment about the desired ZMP on the model used for generating the desired gait. By intentionally generating the moment on the model as above, an effect equivalent to that of generating the moment in the direction of posture recovery on the actual robot can be obtained. In this case, it is also possible to arbitrarily generate the moment on the model without considering the range in which the ZMP can exist. Therefore, even if it is difficult to generate the moment in the direction of posture recovery on the actual robot, generation of the moment on the model enables the posture of the actual robot to be appropriately stabilized in the recovery direction.
In this technology, however, the moment is generated on the model by adjusting translational acceleration in a horizontal direction of the robot's body (eventually, by adjusting a translation floor reaction force's horizontal component acting on the robot). Thus, the robot may slip when it moves on a floor with a low frictional force or when the movement of the robot, such as running, includes a period during which the vertical component of the translation floor reaction force becomes very small (a frictional force which can be eventually generated becomes small).
Therefore, it is an object of the present invention to propose a control device for a legged mobile robot, enabling a robot to move with a stable posture without causing slipping.